This disclosure relates generally to membrane contactor systems for gas-liquid contacting in process industry and, more specifically, to membrane contactors utilizing an oleophobically-treated expanded polytetrafluoroethylene membrane.
Systems for capturing and/or separating liquids and gases are desirable in a variety of applications. Exemplary gas-liquid contacting applications can include carbon dioxide separation, natural gas sweetening, degasification of oil, and the like. For example, the removal of carbon dioxide or other compounds from gases may be desirable or necessary for a number of reasons. If a gas is to be burned as fuel or emitted into the atmosphere as a waste flow, the removal of carbon dioxide from the gas is necessary in order to satisfy the carbon dioxide emission requirements which are set by air pollution control authorities. In natural gas, for instance, removing carbon dioxide (CO2) from the gas can satisfy sales specifications or other process-dependent requirements.
Several systems exist for removing components, such as CO2, from gases. Packed bed scrubbers, distillation columns, strippers, and the like, are all apparatuses used in such gas-liquid contacting applications for separation/removal of components. An example of a removal process using absorption includes removing CO2 from flue gas by means of an aqueous amine solution. The gas to be separated is led into an absorption column where it comes into contact with amine solution, which absorbs the CO2 molecules. The solvent can then be led to a desorption process where the liquid is heated, and the CO2 molecules are removed from the amine solvent by means of a desorption column. The solvent is cooled and passed back to the absorption column, while the concentrated CO2 is removed.
In an absorption column, the amount of contact time with the solvent can determine the degree of purification for the gas. Therefore, a certain liquid surface area per volume must exist for contact with the gas in order to purify the gas. Moreover, the amount of gas which has to be treated factors into the size of the apparatus. An absorption column, therefore, can require a large diameter and height in order to treat a desired amount of gas to the desired purification. Not only does this increase the cost of the system, but it can also impact the system's utility in applications where size and weight are a particularly expensive commodity, such as in offshore installations.
Gas absorption membranes are used as contacting devices between a gas and a liquid flow. These membrane contactors contain a porous membrane, which promotes contact between the liquid and the gas phase. The separation is caused by the presence of an absorption liquid (e.g., an amine solvent) on one side the membrane, which selectively removes certain components from the gas flow (e.g., CO2) from the other side of the membrane. This technology is currently being used as a substitute for the apparatuses mentioned above in gas-liquid contacting applications.
The replacement of the conventional absorption columns with membrane contactors can lead to significant reductions both with regard to cost and weight for a separation system absorption unit. Membrane contactors can provide up to multiple orders of magnitude more surface area per volume than the conventional absorption contactors mentioned above, due to the porous nature of the membrane. Further, membrane contactors are free from problems like channeling and flooding that can occur in packed and tray columns.
Membrane contactors, however, can suffer from some drawbacks. Often times the membrane material, and the porosity thereof, is useful only for a limited range of liquids. Moreover, the pores of the membrane can block over time, thereby reducing the effectiveness of the contactor. Some membranes tend to absorb certain liquids and/or contaminating agents. The materials can clog up the pores of the membrane and prevent the desired gas from diffusing therethrough, or the membrane can no longer effectively resist penetration by the liquid phase.